1. Field of the Invention
The present invention relates generally to high performance epoxy resins that are used in the aerospace industry. More particularly, the present invention relates to improving the flexural strength and strain to failure of such epoxy resins.
2. Description of Related Art
Epoxy resins that are reinforced with a fibrous material, such as glass or carbon fiber, are used in a wide variety of situations where high structural strength and low-weight are required. Composite materials that use a high performance epoxy resin matrix are especially popular in the aerospace industry where weight and structural strength are important engineering and design considerations. High performance epoxy resins may include one or more thermoplastic materials that provide “toughening” of the epoxy resin. In addition, various combinations of curative agents are used to provide optimum curing and resin strength. Although such high performance epoxy resin composite materials are desirable because of their relatively high strength to weight ratio, they do present some specific issues with respect to flexibility and flexural properties. The flexural properties of an epoxy resin are important for design considerations because the overall strength, damage tolerance and resistance to impact of composite parts made using such epoxy resins are dependent upon these properties.
Flexural strength, flexural modulus and strain to failure are flexural properties of a cured epoxy resin that are routinely measured in the aerospace industry. The flexural strength of a cured epoxy resin is defined as its ability to resist deformation under a load. The flexural strength is determined by measuring the amount of force or load that is required to make a cured epoxy resin test specimen fail. For materials that deform significantly without breaking, the load at failure is the point at which the specimen's resistance to bending drops dramatically. The flexural modulus is the ratio of the stress (load) to strain (flexing) during deformation of the cured epoxy resin. The flexural modulus is determined by using the values obtained during testing of flexural strength to calculate the flexural modulus. Strain to failure is a measure of the degree to which a specimen will bend (strain) before it fails.
ASTM D790 and ISO 178 are two standard test procedures that are used to determine the flexural properties of cured epoxy resins. These two procedures are basically the same. A test specimen is supported on a support span and the load (stress) is applied to the center by a loading nose to produce a three-point bending (strain) at a specified rate. The various parameters for the test procedure include the size of the support span, the speed of loading and the maximum deflection for the test. These parameters depend upon the size of the test specimen, which differs between the ASTM D970 and ISO 178 protocols. A common size for the test specimen is 3.2 mm×12.7 mm×125 mm for the ASTM D790 test and 10 mm×4 mm×80 mm for the ISO 178 tests.
The development of high performance epoxy resins where the flexural strength and strain to failure are made as high as possible without deleteriously affecting the flexural modulus has been, and continues to be, a major goal in the aerospace composites industry.
Epoxy resin formulations typically include one or more curative agents. One such curative agent is 4,4′-diaminobenzanilide (DABA). DABA is typically supplied as a powder that is mixed directly with the epoxy resin. It would be desirable to provide epoxy resins that are cured with DABA and which exhibit improved flexural strength and strain to failure without negatively affecting the flexural modulus of the DABA-cured epoxy resin.